Each of these four processes is a basic mechanism of evolutionary change.

Mutation
A mutation could cause
parents with genes for bright green coloration to have offspring with a gene for
brown coloration. That would make genes for brown coloration more frequent in the population than they were before the mutation.

Migration
Some individuals from a population of brown beetles might have joined a population of
green beetles. That would make genes for brown coloration more frequent in the green beetle population than they were before the brown beetles migrated into it.

Genetic drift
Imagine that in one generation, two brown beetles happened to have
four offspring survive to reproduce. Several green beetles were killed
when someone stepped on them and had no offspring. The next generation would
have a few more brown beetles than the previous generation  but
just by chance. These chance changes from generation to generation are known as genetic drift.

Natural selection
Imagine that green beetles are easier for birds to spot (and hence,
eat). Brown beetles are a little more likely to survive to produce offspring.
They pass their genes for brown coloration on to their offspring.
So in the next generation, brown beetles are more common than in the previous generation.

All of these mechanisms can cause changes in the frequencies of
genes in populations, and so all of them are mechanisms of evolutionary
change. However, natural selection and genetic drift cannot operate unless
there is genetic variation  that is, unless some individuals are
genetically different from others. If the population of beetles were
100% green, selection
and drift would not have any effect because their genetic make-up could
not change.